The present invention relates to the oxygen reduction reaction that may occur at the cathode of an electrochemical cell, such as a fuel cell or an electrosynthetic cell. More particularly, it relates to improving the efficiency of such reactions by using a charge transfer mediator that is capable of transferring electrons and protons, in combination with a redox catalyst.
An electrochemical cell is a device comprising two half-cells, each of which comprises an electrode and an electrolyte. In operation, chemical species in one half-cell lose electrons (oxidation) to the electrode (the anode), while chemical species in the other half-cell gain electrons (reduction) from the electrode (the cathode). Each electrode is attached to a structure suitable for transmitting electricity through an external circuit. Furthermore, in order to maintain a compensatory flow of charge within the cell, certain ions may be allowed to move freely between the two half-cells (i.e., the two half-cells are in “ionic communication” with each other).
In one type of electrochemical cell, electrical energy is generated by the spontaneous reaction occurring at each half-cell (together, a redox reaction). Such cells are sometimes called voltaic or galvanic cells. In fuel cells, which are a specific type of galvanic cell, a fuel (e.g. hydrogen, methanol, methane, or another material that can be readily oxidized) is oxidized at the anode, and oxygen or another oxidizing agent is reduced at the cathode, generating a flow of electricity that can be used commercially in a variety of applications, such as for generating primary or backup electrical power, or to supply the electricity needed to run an electric vehicle, such as a forklift or an automobile.
In another type of electrochemical cell, known as an electrosynthetic cell, a desired chemical product is synthesized at the anode or cathode. Such cells may, like voltaic cells, generate electricity as a byproduct of the desired synthesis, or alternatively, they may require that electrical current is continuously applied to drive a non-spontaneous redox reaction. As an example of an electrosynthetic cell, methanol may be oxidized at the anode to produce formaldehyde, and oxygen or another oxidizing agent may be reduced at the cathode. Note that in operation, the same oxygen reduction reaction may occur at the cathode in either a fuel cell or an electrosynthetic cell.
Platinum cathodes are widely used to facilitate oxygen reduction in cathodic half-cells. Although quite efficient, such cathodes are very expensive, due to platinum's relative scarcity. To render such cells more practical, it is desirable to use lower cost cathodes, without materially compromising efficiency, useful life, or other desired characteristics.
To address these concerns, there have been a variety of efforts to develop improved cathodic half-cells for oxygen reduction. Some have attempted to surface coat base cathode materials, thereby reducing the amount of the most expensive metals that need to be used.
In WO 2012/085542, it was proposed that a polyoxometalate redox catalyst could be used in conjunction with a vanadyl mediator to facilitate electrocatalytic oxygen reduction using a less expensive cathode. While this had some benefits, further improvements were still desired.
There remains a need for improved half-cells for more efficient electrocatalytic oxygen reduction, particularly with respect to avoiding the need for expensive metals to efficiently catalyze the oxygen reduction reaction that may occur at the cathode of an electrochemical cell.